1. Field of the Invention
This invention relates generally to nematic liquid crystal compositions and more specifically to mixed liquid crystal compositions including a P,P'-dialkylazoxybenzene as the main component. The compositions also include at least one component selected from an ester liquid crystal, a cyclohexanecarboxylate liquid crystal and a phenylcyclohexane liquid crystal. The mixed liquid crystal compositions have excellent multiplex characteristics and may be driven by a multiplex drive at a duty ratio of 1/64 or less, at a driving voltage of 15 V or less.
2. Description of the Prior Art
The known methods for driving TN-type liquid crystal display devices include static drive and multiplex drive. The multiplex drive is more advantageous for several reasons. Because the required number of leads can be reduced, (1) the structure of a liquid crystal cell can be simplified, (2) the number of joint parts between the liquid crystal cell and a drive circuit output members can be reduced and (3) the drive circuit can be simplified. In addition, when it is intended to provide a display of enlarged capacity, the required number of leads is increased further, and therefore, multiplex drive at a low duty ratio is required.
When the multiplex drive is used, there has been a problem in that the driving voltage is restricted depending on the liquid crystal material. For example, a selective wave-form of a multiplex drive wave-form is shown in FIG. 1(a) and a semi-selective wave-form thereof is shown in FIG. 1(b). The relationship between the voltage and the light transmittance of the liquid crystal cell corresponding to this wave-form is shown in FIG. 2. Graph 1 and Graph 2 show the relationship where the angle between the liquid crystal cell and the direction of observation (the viewing angle) is 80.degree. when the selective wave-form (a) and the semi-selective wave-form (b) are applied to the liquid crystal cell, respectively. Vth-1 and Vth-2 represent the threshold voltages at light transmittance of 30% and 80%, respectively. At that time, the driving voltage range in which the multiplex drive can be performed with sufficient contrast is: EQU Vth-1.ltoreq.Vd&lt;Vth-2
On the other hand, the driving voltage can fluctuate due to a change in the battery voltage with time, scattering in the circuit and other reasons. Therefore, as a measure which allows for the fluctuation in the driving voltage, a voltage margin (M) is defined by the following equation: ##EQU1## wherein Vc is a driving voltage: EQU Vc=1/2[(Vth-2)+(Vth-1)]
and thus, demonstrates suitability of the liquid crystal in multiplex drive.
However, since the smaller the duty ratio in general, the lower the ratio of the effective voltage of the selective wave-form to that of the semi-selective wave-form, the difference between Vth-1 and Vth-2 decreases and so does the margin (M). Further, if the duty ratio is lowered more, then the following are obtained: EQU Vth-1&gt;Vth-2, and M&lt;0
at which duty ratio, multiplex drive is not possible as a practical matter. Therefore, in order to provide a display having a larger capacity and high quality, a liquid crystal material to be driven by a multiplex drive at a smaller duty ratio with a wider voltage margin is required than is available.
Based on the above, there are known liquid crystal compositions including the following liquid crystal compounds: ##STR3## wherein R represents various straight-chain alkyl groups. However, it has been extremely difficult to provide a mixed liquid crystal composition which may be driven by a multiplex drive at a duty ratio of less than 1/16 by combining the above liquid crystal compounds. Therefore, it is desirable to provide an improved liquid crystal composition suitable for use in large capacity liquid crystal display devices which will overcome these shortcomings of the prior art.